1. Field of the Invention
The invention relates to a nonaqueous electrolyte battery, an electrode plate for the nonaqueous electrolyte battery, a method for manufacturing the electrode plate for the nonaqueous electrolyte battery, and an apparatus for manufacturing the electrode plate for the nonaqueous electrolyte battery.
2. Description of the Related Art
A nonaqueous electrolyte battery having lithium as an anode active material is attracting attention as a high-energy density battery, and particularly a primary battery having manganese dioxide, carbon fluoride or thionyl chloride as a cathode active material is extensively used as power supply for a calculator or a timepiece and as a backup battery for a memory.
Besides, with miniaturization and weight reduction of a camcorder, a laptop computer, a portable phone and other various electronic equipment in these years, demands for a secondary battery with a high-energy density as a power supply for such equipment are increasing, and a lithium secondary battery having a carbon material as anode active material is being studied vigorously.
Batteries having a nonaqueous electrolyte with a main component of an organic electrolytic solution (hereinafter referred to as the nonaqueous electrolyte battery) which are represented by a lithium battery need to have a thin electrode plate because electric conductivity of the nonaqueous electrolyte is lower than an aqueous electrolyte. And, since a large reaction area is needed to obtain a large current, the cathode and anode plates are made into a sheet, and these electrode plates are wound into a roll through the intermediary of a separator to form a spiral structure. The electrode plate for such a structure is a coated type electrode which is obtained by coating a conductive base material with a coating liquid which contains an active material and a conductive material which are called as the mix.
FIG. 13 shows one example of such a battery structure.
The battery shown in FIG. 13 has a cathode plate 1 and an anode plate 2 laminated with a separator 3 which is an insulative film having a high ion transmittance therebetween, wound into a roll and housed in a container 4. And, a positive tab plate 5 is connected to a current collector at the inner periphery end of the cathode plate 1, and the positive tab plate 5 is further connected to a positive terminal 6 through a cap 6a. A negative tab plate 7 is connected to a current collector at the outer periphery end of the anode plate 2, and the negative tab plate 7 is also connected to a negative terminal 8 at the bottom of the container 4.
The cathode plate 1 and the anode plate 2 are produced by cutting to a desired length a sheet electrode plate which has both surfaces of a conductive base material coated with an electrode mix (electrode material coating agent) which mainly consists of a cathode or anode active material, a conductive agent and a binding agent. As shown in FIG. 14A to 14C, to connect the positive tab plate 5 and the negative tab plate 7 to the cathode plate 1 and the anode plate 2 respectively, it was conventional that a cross-shaped incision was formed in an electrode plate 9 (cathode plate or anode plate) and a tab plate 10 (cathode tab plate or anode tab plate), and the tab plate 10 was folded backward and crimped. But, a battery which had the tab plate 10 connected by this method had an unstable impedance. And, there were also disadvantages that the crimped part became thick and the tips of crimped part 11 ruptured the separator inducing a short circuit.
And, the tab plate is also connected to the electrode plate by welding, but the welded part on the electrode plate cannot be coated with an electrode mix to be the base of the conductive base material.
To expose the base of the conductive base material, it was conventional to remove the electrode mix layer which was formed by coating and drying an electrode material coating liquid. For example, Japanese Patent Publication No. Sho 60-48865 discloses a method of removing the electrode mix layer by forming lines with a pair of knife-edges and scraping the electrode mix layers between the lines by a screw slotting cutter. And, Japanese Patent Laid-Open Publication No. Hei 2-98040 proposes a method of removing the electrode mix by contacting blades from top and bottom to the base material, both surfaces of which are coated with the electrode mix layer. But, these methods needed the extra work of removing the electrode mix and wasted the removed electrode mix.
On the other hand, there is a method of coating the electrode mix with the base of the conductive base material left uncoated. Specifically, there is proposed a method of manufacturing a sheet electrode plate wherein a conductive agent is mixed with an electrode active material, a binding agent or the like is added thereto to prepare a paste electrode material (electrode mix) coating liquid, and this coating liquid is alternately applied and not applied to the surface of a conductive base material.
And, to produce the sheet electrode plate for electrodes as described above, there have been proposed a method of force charging a conductive mix which has a conductive agent and a binding agent kneaded with an electrode active material into a supporting material (conductive base material) while rolling, a method of extruding the kneaded electrode mix to form on both sides of a supporting material (Japanese Patent Laid-Open Publication No. Hei 4-282558), a pickup method (Japanese Patent Laid-Open Publication No. Sho 62-256365, Japanese Patent Laid-Open Publication No. Sho 63-114058), a pull-down method (Japanese Patent Laid-Open Publication No. Hei 1-267953, Japanese Patent Laid-Open Publication No. Hei 1-194265), a reverse-roll method, a gravure-roll method, a doctor blade method, and a method which uses an extrusion type injector having a slot nozzle (Japanese Patent Laid-Open Publication No. Hei 7-65816), to continuously coat the supporting material with the electrode mix.
For example, an electrode mix uncoated area may be formed in a direction that the conductive base material is moving by the roll coating such as a gravure roll method or a reverse roll method. For example, a reverse-roll method, which transcripts a coating liquid to the conductive base material by a backup roll, separates the backup roll from a coating roll to form an uncoated area in a longitudinal direction of coating. But, since the coating roll is revolving while the backup roll is separated from the coating roll, the coated thickness becomes excessively thick when the backup roll comes in contact again with the coating liquid on the coating roll. To prevent it from occurring, the coating roll may also be stopped at the same time when the backup roll is separated, but this method is not so effective although some improvement is achieved. Besides, the coating liquid on the roll is dried unevenly, causing bulges at the start and end of coating. Thus, the coated layer does not have a uniform thickness, having poor surface smoothness.
Besides, there is also proposed a manufacturing method that an electrode material coating liquid is injected by an extrusion type injector having a slot nozzle and coated onto the running conductive base material with uncoated areas formed parallel to the running direction (longitudinal direction), thereby producing a sheet electrode plate (Japanese Patent Laid-Open Publication No. Hei 7-94170). This method is a continuous coating method which can form a good coated layer while continuously disposing a longitudinal stripe-like uncoated area where the electrode mix is not formed.
This method, however, needs extra steps that the slot nozzle is exchanged to change the position and width of the uncoated area and a plate which is tightly fixed to the slot is moved to divide an opening.
And, there is proposed a method that the electrode material (electrode mix) coating liquid is excessively supplied to a conductive base material in advance, and the coating liquid is paused from being supplied by a shutter which is disposed just before a doctor blade, thereby forming uncoated areas at predetermined intervals in a running direction (longitudinal direction) of the conductive base material. But, even if the coating liquid is paused from being supplied by closing the shutter, the coating liquid left on the doctor blade adheres often to the uncoated areas. Therefore, this method has disadvantages that when a tab plate is connected to the uncoated area by welding, the adhered electrode mix deteriorates a welding strength of the tab plate, and the tab plate is easily separated.
Thus, the method which removes the coated and dried electrode mix layer to expose the base of the conductive base material in order to weld the tab plate needs an extra step and cannot produce the sheet electrode plate efficiently.
The method which forms the electrode mix uncoated areas by means of the shutter disposed immediately before the doctor blade has disadvantages that the electrode mix adhered to the uncoated areas degrades a weld strength of the tab plate and separates the welded tab plate. Besides, the method of forming the uncoated areas by the roller coating causes bulges at the start and end of coating, possibly cutting the conductive base material while pressing.
Specifically, the sheet electrode plate is compressed under high pressure by the roller press. But, when a pressure is raised, a space (gap) between the press rollers becomes substantially zero if no conductive base material is between them. Therefore, if the sheet having the electrode mix uncoated areas is pressurized and compressed under this situation, a very strong force is applied when the uncoated area is passed between the rollers after the coated area, and the conductive base material is partly stretched or cut. And, if the conductive base material is cut off, the opposed rollers are directly contacted mutually, resulting in damaging the roller surfaces.
Thus, batteries using the sheet electrode plates produced by the above methods tend to suffer from deterioration in performance of charging and discharging cycle over a long period.
Specifically, these methods had disadvantages of needing additional processes of changing nozzles and peeling because the supporting material is continuously coated with a predetermined amount of electrode mix. The sheet electrode plates produced by the above methods are cut to a length suitable for a single cylindrical or square battery and wound into a roll, and when it is particularly used for a cylindrical battery, inflow and outflow of the electrolytic solution owing to the charging and discharging are different between the core and the outer periphery due to a difference in radius of curvature, and performance is easily degraded by a charging and discharging cycle over a long period.